Composition for hydraulically fracturing formations



COMPOSITION FOR HYDRAULICALLY FRACTURING FORMATIONS Harold W.Brainerd,Jr., Overland Park, Kans., Harold W. Gray, Jr., Odessa, Tex.,and Paul L. Menaul, Tulsa,

kla., assignors to Pan American Petroleum Corporation, a corporation ofDelaware i No Drawing. Application February 28, 1956,

Serial N0. 568,165

8 Claims. (Cl. 252-855) This invention pertains to an improvedcomposition of matter. 7 More specifically, this invention pertains to atemporary oil-in-water emulsion which is particularly adapted to be usedas a fracturing fluid in the Hydrafrac process. This is acontinuation-in-part of our U. "S. application Serial Number 344,242,filed March 23, 1953, now abandoned.

The Hydrafrac process, as disclosed in Re. 23,733, Farris, is a processfor producing fractures in a subterranean formation by the applicationof a high hydraulic pressure within a well at the elevation of theformation which is to be fractured. It has been found highly desirablethat the fracturing liquid have a high initial viscosity and lowfiltrate rate, and that eventually the viscosity be broken back ordecreased preferably to a viscosity Within the range of the well fluids.The viscosity of the fracturing fluid is typically in the range of1,0005,000 centipoises or greater. With fluid of this viscosity, agranular propping agent is carried by the fluid into the fracture, andadditionally the filtration of the fracturing liquid into the formationis sulficiently retarded so that a high pressure can be applied to theformation by injecting the fracturing fluid into the well at areasonable pumping rate. It is also generally considered highlydesirable that the fracturing liquid should not contaminate thefractured formations with a fluid which might produce a fluid blocktherein or a decrease in formation permeability. That is, in wellsproducing oil, particularly where the oil is producedgfrom apreferentially water-wet formation, contamination of the formation withwater is considered undesirable. Furthermore, in many limestone andsandstone formations substantial quantities of hydrophilic colloids havebeen found which, if contacted with a relatively fresh water, tend toswell, thereby decreasing the formation permeability. The deleteriouseffect of these contaminants often overcomes the advantages obtainedthrough the increasing of permeability by the Hyd-rafrac process.Hydrocarbon gels including particularly hydrocarbons gelled with variousoil-soluble soaps have generally been used to produce fractures inoil-producing for-v mations. The cost of producing such gels is veryhigh, particularly in col-d climates, since the gels can be producedquickly and efiiciently only at temperatures above about 70 F.

'11: is an object of this invention to provide an improved andrelatively cheap fracturing liquid which is initially veryviscous andwhich automatically reduces to a highly fluid state after a formationfracture has been produced. It is a more specific object of thisinvention to provide a viscous oil-in-acid emulsion containing a veryminpr amount of water, the emulsion being produced readily at anytemperature with an ester which is hydrolyzed by the acid in theemulsion whereby. the emulsion is broken and the extraneous liquids canbe removed from the formation as the well is produced. These and otherobjects of this invention will become apparent from thefollowing'detailed description.

:Our improved composition of matter comprises in brief a majorproportion of a light hydrocarbon liquid emulsified in-a very minorproportion of an acid solution with Patented May 28, 1 257 awater-soluble nonionic surface active agent of the ester type. With thisemulsifying agent the concentration of the hydrophilic constituent ofthe emulsion may be so small that the contamination and reduction ofpermeability in the formation is substantially negligible. The waterphase, however, contains suificient acid to hydrolyze the emulsifier sothat after a suitable time delay which permits the fracturing liquid tobe injected into a formation, the tfilm of surface active agent betweenthe hydrophilic and lipophilic components is destroyed, and the emulsionis thereby broken, allowing the oil droplets to agglomerate whereby theliquid components of'the emulsion can be removed from the pores of theforma tion with the connate liquids.

Thehydrocarbon or oily component of the emulsion fracturing fluid istypically a hydrocarbon in the boiling range of gasoline, kerosene,naphtha, light fuel oil, light crude oil, and the like, and mixtures ofthese hydrocarbons. Heavier hydrocarbons can, of course, be used. Forexample, viscous oils in the motor oil range and heavy crude oil producesatisfactory emulsions, but, due to their greater viscosity, the removalof such viscous oils from a formation after a fracture is produced issomewhat more diflicult than the removal of these lighter hydrocar-bons.That is, since the liquids in the emulsion return to their initialviscosities after the emulsion is broken, it is considered desirable,but not necessary, that the liquid components of the emulsioncomposition have an initial viscosityin the lighter hydrocarbon range,typically in the range of about l-locentipoises. Crude oils are lessdesirable than refined oils due to the fact that many crude oils containnaturally an'emulsifying agent which may tend to stabilize theemulsion.Kerosene is. the preferred light hydrocarbon in'view of its wideavailability, its relatively uniform composition, etc.

This light hydrocarbon is emulsified with an acid solution. Any acidwhich will hydrolyze an ester-type emulsifying agent is consideredsatisfactory. For example, hydrochloric, hydrofluoric, or other strongmineral acids which may be injected into a formation in solutions whichare immiscible with the hydrocarbon phase, or an organic acid such asacetic acid may be employed. Hydrochloric acid is preferred. These acidsare typically used in water solutions. The concentration of the acid maybe varied over a substantial range. Since the function of the acid perse is largely to hydrolyze the emulsifying agent Within a reasonabletime, the concentration of the acid may be adjusted by experimentationso that the emulsion will be broken in the desired time, say within 24hours. In the preferred embodiment using a hydrochloric acid solution,the concentration of the acid in the water solution is normally betweenabout 5 and about 25 percent. A 15 percent hydrochloric acid solution,in view of its wide use in producing operations, is usually employed inshallow wells. The following Table I shows the elfect of acidconcentration and temperature on the breakdown time of the emulsion.Breakdown time is the time required for percent of the hydrocarbon toseparate from the emulsion in a beaker.

The amount of the acid solution or water phase, as indicated above, ispreferably maintained at a minimum since many formations arecontaminated and the permeability is decreased by the presence of theacid solution .and/or its derivatives. For this reason, between'onlyabout 2 and about 15 percent of the acid solution based on the volume ofhydrocarbon is normally used. The acid solution is preferably betweenabout 5 and percent of the volume of the hydrocarbon.

-The light'hydrocarbon is emulsified with the immiseible aqu'eous'phaseby use of a water-soluble nonionic surface active agent comprising apolyoxyethylene anhydrosorbitol monolaurate containing at least 12ethylene oxide groups per molecule. A surface active agent of this type,known as Tween 20, is manufactured by Atlas Powder Company of,Wilmington, Delaware. This surface active agent is produced by firstdehydrating a sorbitol to produce a hexitan. 1 The hexitan is thenesterified with lauric acid to form the monolaurate which is treatedwith ethylene oxide to add polyoxyethylene chains to the non-csterifiedhydroxyls. Sufficient ethylene oxide groups are added to thenon-este'rified hydroxyls to make the surfactant highly water soluble.Between about 12 and about ethylene oxide groups per molecule may beemployed. About 20 ethylene oxide groups are preferred. This surfactantis highly soluble in highly ionized solutions such as strong acidsolutions and is not soluble in the hydrocarbon phase. Being highlywater soluble and oil insoluble, the amount of this surfactant useddepends generally upon the amount of acid solution used. Between about 1percent and about 10 percent of surfactant based'upon the volume of theaqueous phase or acid solution is normally used. About 2 percent byvolume is preferred.

The emulsion is produced by first dissolving or dispersing the surfaceactive agentin the aqueous phase. A small amount of hydrocarbon is thenadded to this aqueous phase by rapid stirring or agitation to produce aprimary emulsion. Typically about equal volumes of hydrocarbon phase andaqueous phase are first emulsified to produce the primary emulsion.After a primary emulsion is produced, the remainder of the hydrocarbonphase is added by the batch method as by adding the remainder of the oilto the pump suction as the primary emulsion is circulated until all ofthe oil and aqueous phases are emulsified or by the continuous method byadding the remainder of the oil to the pump suction, preferably acentrifugal pump, as the secondary emulsion or fracturing fluid isinjected into the well. The viscosity of this emulsion depends somewhatupon the method of mixing, the temperature, the viscosity of thehydrocarbon phase, and the like, but in general the viscosity istypically in the range of LOGO-5,000 centipoises or greater as measuredon the Gardner Mobilometer.

The viscosity of the resulting emulsion may also be varied by varyingthe ratio of the aqueous and nonaqueous phases, the viscosity of thenonaqueous phase, or by varying the concentration of the surfactant.Laboratory or pilot runs are sometimes made to adjust the viscosity andbreakdown rate as desired.

While the filtrate rate of the above-described emulsion is acceptableand the viscosity is sufficient to support propping agents such as 40-60mesh U. S. sieve sand which is normally added before the fracturingliquid is injected into the well, its filtrate rate or filter loss tothe formation may be improved by the addition of finely divided solids.gums, e. g., locust bean gum, karaya gum or preferably batu gum, groundto pass through a 100 mesh sieve can be dispersed separately or togetherwith the propping agent in the fracturing liquid before that liquid isin jected into the well. Any amount of the solids may be added so longas the concentration is maintained low enough to permit the fracturingliquid to be pumped. Typically between about land about 7 pounds each ofVarious solids such as the sand and of gum are added per barrel offracturing liquid. About 2 pounds of each are preferred.

As an example of the characteristics of a typical fracturing fluid ofthe typeabove described, parts by weight of kerosene were emulsified in5 parts of 10 percent hydrochloric acid solution using l part of water-'soluble nonionic polyoxyethylene anhydrosorbitol mono laurate (Tween20). This emulsion was then pumped through a 1-inch diameter pipe nipplecontaining a 6- inch section of 40 mesh Ottawa sand in a simulated 6,000foot well test at F. The viscosity of the emulsion both before passingit through the'sand and subsequently was greater than about 5,000centipoises, this viscosity being measured by comparison with theviscosity of hydrocarbon soap gels in a falling-ball consistometer. Thuswhere an emulsion is generally broken by intimate contact'with a sandbed, the Water-soluble polyoxyethylene anhydrosorbitol monolaurate bycomparison produced an emulsion with a high oil-to-water ratio which, asindicated, passed through the sand bed without being materiallyaffected. Due to the hydrolysis of the emulsifying agent by the weakacid solution, the emulsion was substantially completely broken withinabout two hours and the viscosity was reduced to substantially theviscosity of the oil (less than 10 centipoism). Within less than a daythe aqueous and hydrocarbon phases had completely separated and theviscosity of each component was, of course, reduced to its initialviscosity.

From the foregoing, which is given by way of example, it can be seenthat our improved composition is particularly adapted to be used in theHydrafrac process. It

is, however, susceptible of a wide variety of uses where temporaryoil-in-water emulsion gels are desired. Accordingly, this inventionshould be construed to be limited only by the scope of the appendedclaims.

We claim:

1. An oil-in-water type emulsion comprising a minor proportion of astrong mineral acid, a major proportion of a light hydrocarbon and awater-soluble nonionic surface active agent having an ester linkagehydrolyzable by the action of said acid to break said emulsion.

2. A viscous fluid for fracturing a formation comprising a majorproportion of a light hydrocarbon emulsified as the disperse phase in aminor proportion of an acid solution, and a water-soluble nonionicsurface active agent having an ester linkage hydrolyzable by the actionof said acid solution to reduce the viscosity of said fluid after a timesufiicient to permit said fluid to be withdrawn from said formation. a

3. A viscous fluid for fracturing a formation comprising alight-hydrocarbon emulsified as a disperse phase in a quantity of astrong mineral acid solution, said quantity comprising between about 2and about 10 per cent by volume of said light hydrocarbon, and awater-soluble nonionic surface active agent having an ester linkagehydrolyzable by the action Of'said acid to reduce the viscosity of saidfluid after a time sufiicient to permit said viscous fluid to beinjected into said formation. 7

4. A viscous fluid for fracturing formations comprising a quantity of alight hydrocarbon emulsified asthe disperse phase in a quantity of awater solution of astrong mineral acid, said solution being betweenabout '2 and about 15 per cent by volume of said light hydrocarbon andthe concentration of said acid being between about 5 and about 25percent in said water solution, and a quantity of a water-solublenonionic surface active agent having an ester linkage hydrolyzable bythe action of said acid'to reduce the viscosity of said fiuid after atime suificient to permit said viscous fluid to be'injected'into saidformation.

5. A viscous fluid in accordance with claim 4 wherein said quantity ofwater-soluble nonionic surface active agent is'between about 1 percentand about 10 percent of the volume of said water solution of a strongmineral acid.

16. A viscous fluid for fracturing a formation comprising by volume 100parts of a light hydrocarbon emulsified as the disperse phase in about 5parts of a 10 percent hydrochloric acid solution, said emulsion beingproduced and stabilized by a water-soluble nonionic anhydrosorbitolmonolaurate having between about 12 and about 25 ethylene oxide groupsper molecule.

7. A viscous fluid in accordance with claim 6 in which saidwater-soluble nonionic anhydrosorbitol monolaurate has about 20 ethyleneoxide groups per molecule.

8. A viscous fluid in accordance with claim 7 in which the quantity ofsaid monolaurate is between about 1 and about 10 percent of the volumeof said acid solution.

5 References Cited in the file of this patent UNITED STATES PATENTSBlair et al Aug. 22, 1944 Lehmann et a1 Aug. 22, 1944 Brainerd Apr. 17,1956

1. AN OIL-IN-WATER TYPE EMULSION COMPRISING A MINOR PROPORTION OF A STRONG MINERAL ACID, A MAJOR PROPORTION OF A LIGHT HYDROCARBON AND A WATER-SOLUBLE NONIOMIC SURFACE ACTIVE AGENT HAVING AN ESTER LINKAGE HYDROLYZABLE BY THE ACTAION OF SAID ACID TO BREAK SAID EMULSION. 